Tree-resistant ethylene polymer compositions

ABSTRACT

Propagation of electrical trees and water trees in electrical insulation made of ethylene homopolymers or copolymers with an unsaturated monomer is inhibited by the addition to the insulating composition of an organic carboxylic ester having at least one aromatic ring and at least three carboxylic ester groups, the ester being liquid at the operating temperature of the electrical equipment in which the insulation is used. The main utility of the inhibitors of this invention is in the primary insulation for power transmission cables, especially those used in high voltage applications.

BACKGROUND OF THE INVENTION

This invention relates to ethylene polymer compositions especiallyuseful in making cable insulation for high voltage applications. Thecomposition contains an additive which provides resistance to electricalbreakdown.

Electrical breakdown of high voltage insulation, known as dielectricfailure, is often initiated at the sites of cavities and contaminatingparticles. Despite extreme care used in making, handling, and extrudingethylene polymer insulation, cavities and contaminants can be introducedin any step prior to final shaping. The breakdown of insulation in highvoltage applications is known to the trade as electrical "treeing".Electrical treeing is a rather slow progressive degradation of aninsulation composition caused by electron and ion bombardment of theinsulation and resulting in the formation of microchannels or tubeshaving an overall tree-like appearance. Trees are initiated at locationsof cavities or contaminants by the action of ionization (corona) duringhigh voltage surges. Once initiated, trees usually grow, hastened byvoltage surges, until such time as dielectric failure occurs.

Another phenomenon which may cause electrical breakdown is watertreeing. Water trees are different in appearance from electrical trees.They have a diffuse and indistinct appearance; they do not appear to bebranched or to be channels. They are believed by some researchers to bemicrocracks or minute water agglomerates. They are found only in cablesthat have been exposed to normal operating electric stresses in a moistor wet environment. Water trees, like electric trees, are initiated atcavities and contaminating particles. It has been suggested that aswater trees grow, they can become electrical trees as part of theultimate electrical breakdown.

To overcome the problem of treeing, various additives have beenproposed, particularly for use in polyethylene or polyolefins, whichadditives serve to either prevent formation of trees or delay treegrowth. Certain alcohols have been found to be very effective additivesfor delay of tree growth in ethylene polymer insulation. See U.S. Pat.No. 4,206,260 to E. J. McMahon. However, the alcohol content decreaseswith exposure of the insulation to elevated temperatures. Exudation ofalcohol can be mitigated but not prevented by addition of polypropylene,as described in U.S. Pat. No. 4,283,459 to Urban et al.

Other additives which have been proposed for ethylene polymer insulationto increase resistance to electrical breakdown include: an inorganicsalt of a strong acid with a strong zwitterion compound in U.S. Pat. No.3,499,791 to Maloney; a ferrocene compound with a substituted quinolinecompound in U.S. Pat. No. 3,956,420 to Kato et al.; silicone fluid inU.S. Pat. No. 3,795,646 to McKenzie, Jr.; and an aromatic ketone inJapanese Pat. No. 14348/75 to Fujikura Cable Works, Ltd.

SUMMARY OF THE INVENTION

According to the present invention, there is now provided a treeresistant composition for use in insulation for high voltage powertransmission cables, said composition consisting essentially of anethylene polymer selected from the group consisting of ethylenehomopolymers and copolymers of ethylene with at least one otherethylenically unsaturated monomer, ethylene being present in suchcopolymers in an amount of at least 85 weight percent, and, as atree-growth inhibitor, an effective amount of at least one organiccarboxylic ester having at least one aromatic ring and at least threecarboxylic ester groups, said inhibitor being liquid at the powertransmission cable's intended operating temperature.

There also is provided an insulated cable for the transmission ofelectric power comprising at least one metallic conductor surrounded byelectrical insulation containing at least one layer made of the abovecomposition.

BRIEF DESCRIPTION OF THE DRAWING

The drawing shows the results of a water tree test for a control sampleand a test sample.

DETAILED DESCRIPTION OF THE INVENTION

Power transmission cables which operate above about 4000 V, areparticularly susceptible to tree formation, either by corona dischargeor by external electrical disturbances. A high voltage powertransmission cable usually comprises a metallic conductor surrounded bya semiconductive layer, an insulating layer, and another semiconductivelayer. The effective amount of a tree-growth inhibitor of the presentinvention in the insulating layer is about 0.25-5% based on the weightof ethylene polymer. The preferred concentration of the inhibitor is1-3%. The inhibitor may migrate from one layer to another and thus itsconcentration in the insulating layer may decrease below its originalconcentration. In order to avoid depletion of the inhibitor in theinsulating layer, it may be practical to also incorporate initially someinhibitor in the semiconductive layers, which normally are ethylenehomopolymer or copolymer compositions containing a form of elementalcarbon, such as carbon black or graphite, as a filler. Alternatively,the initial ethylene polymer composition to be used in fabricating theinsulating layer can be compounded with an excess of tree-growthinhibitor, so that after partial migration into the semiconductivelayers, the concentration remaining in the insulating layer will bewithin the effective range. The inhibitor preferably is added to moltenpolymer since a good homogenous blend is thus obtained the most readily.Other methods of combining the ester with the polymer include, forexample, mixing with solid polymer prior to compounding and diffusing aliquid ester or a solution of ester in a volatile solvent into thepolymer by spraying or soaking.

The ethylene polymer is either a homopolymer or a copolymer with atleast one other monomer. Representative suitable other monomers includeα-olefins such as, for example, propylene, 1-butene, 1-hexene, 1-octene,and 1-decene; butadiene, styrene, methacrylic acid, vinyl acetate, ethylacrylate, isobutyl acrylate, and methyl vinyl ether. Both homopolymersand copolymers of ethylene are well known in the art and many arecommercially available. They may be either linear or branched, highpressure or low pressure types, made in the presence of a free radicalgenerator or with a coordination catalyst.

The ethylene polymer most likely to exhibit significant improvement inelectrical endurance as a result of the addition of an inhibitor of thepresent invention is the low density polymer, that is, one which has adensity of about 0.92 g/cm³ or less. Medium density polymers, in thegreater than 0.92 and up to about 0.94 g/cm³ range, are improved to alesser degree, while the high density polymer, greater than 0.94 g/cm³,exhibits the least improvement. In any event, a tree-growth inhibitor ofthis invention does not inhibit tree initiation but only the rate ofgrowth of trees after initiation.

The insulating compositions of the present invention will, in additionto the inhibitor, also contain other usual compounding ingredients, suchas processing aids, antioxidants, and optionally curing agents (forexample, peroxy compounds). Polypropylene and propylene/ethylenecopolymers are suitable processing aids. The total amount of processingaids may be as much as 10 percent of the weight of the base ethylenepolymer. Low molecular weight polyethylene and wax also may be added.Carbon black normally will be present in the compositions forming thesemiconductive layers but not in the insulating composition.

The tree-growth inhibitors of the present invention most suitably areesters of polycarboxylic aromatic acids, especially mellitic, trimesic,hemimellitic, trimellitic, and pyromellitic acids. Simple esters ofaromatic dicarboxylic acids, for example phthalic or terephthalic acids,are not useful. Other possible esters include those where one or morecarboxylic ester groups are attached to an aromatic ring and theremaining ester groups are attached to an aliphatic radical, forexample, 3,5-dicarboxyphenylacetic acid esters; or where one or morecarboxylic ester groups are attached to one aromatic ring and theremaining ester groups to another aromatic ring fused to the first, forexample, 1,4,6-naphthalenetricarboxylic acid esters; or joined to it bya single bond, an alkylene group (especially a methylene group), acarbonyl group, or a hetero atom (especially oxygen or sulfur). Theseinclude esters of 2,4,4'-biphenyltricarboxylic acid,methylenebis(phthalic acids), and the corresponding acids in whichcarbonyl, oxygen, or sulfur replaces the methylene group. Such esterscan be made by known methods. Their most important requirement is lowmelting point, which preferably is below the intended operatingtemperatures of the high voltage cable in which they are used. Theseesters preferably should be liquid at room temperature, but thosemelting below 50° C. are useful in most applications. Mixtures of two ormore esters can be used, and their melting temperatures normally will belower than those of the individual esters. However, particularly usefulesters are mixed esters, obtained by esterifying a polycarboxylic acidwith a mixture of alcohols, because their melting temperatures are lowerthan those of the corresponding single alcohol ester blends. Thepreferred esters are those in which the alcohol portion is aliphatic,araliphatic, or cycloaliphatic, especially having 4-12 carbon atoms.However, esters of alcohols having either fewer or more carbon atoms,for example, 2 to 18, may be used.

Suitable alcohols thus include, for example, methyl, ethyl, variousisomers of butyl, phenyl, hexyl, heptyl, octyl, decyl, dodecyl,hexadecyl, octadecyl, cyclohexyl, cycloheptyl, and benzyl alcohols.

Also suitable are oligomeric esters of dicarboxylic aromatic acids withaliphatic diols, especially diols having at least six carbon atoms andpreferably at least eight carbon atoms, including various polyglycols.Suitable acids are, for example, terephthalic and isophthalic acids andmethylenebis(benzoic acid). Suitable alcohols include, for example,1,6-hexanediol, 1,8-octanediol, mixtures of these diols, andpoly(ethylene glycol) and poly(propylene glycol) having number averagemolecular weights of about 500 to 2000.

The cable insulating compositions of the present invention are extrudedin a conventional manner and can be cured, if desired, either by heatingto the decomposition temperature of any free radical generator (e.g.,peroxy compound) incorporated therein or by high energy radiation, forexample, with an electron beam.

The electrical endurance of the insulating compositions of the presentinvention is determined in an accelerated test. Although the test isvery useful in comparing the effectiveness of various tree-growthinhibitors, it does not directly predict the actual life expectancy ofinsulation in use. The following electrical tree test method is used.

Ethylene polymer for testing in accordance with this method is initiallymolded into a block herein termed a "SPINGS" (which is an acronym for"solid phase internal needle gap specimen"). A SPINGS is 25 mm square by6 mm thick and contains two electrodes embedded lengthwise and in line,equidistant from the faces and from the opposite edges, with the tipsusually spaced 4 mm apart but sometimes 2 mm apart at the center of theblock. Each electrode is about 30 mm in length and about 0.6 mm indiameter. One electrode has a cone-shaped point at a 30° included anglewith a radius of 5 μm and is the high voltage electrode. The secondelectrode has a 0.3 mm hemispherical radius on one end and is the groundelectrode.

A minimum of ten SPINGS are used in this test. Each SPINGS is placedunder silicone oil, thus, preventing surface flashover. The high voltageelectrode is connected to a high voltage bus and the ground electrode isconnected to a spaced pair of 6.25 centimeter spheres connected toground through a 1 megohm resistor. A gap is set sufficiently widebetween the spheres to achieve a voltage sufficient to initiate a treein the SPINGS. For example, with the spheres set at 0.762 centimetergap, a voltage (60 HZ) increasing at a rate of 500 volts/sec is applieduntil a discharge occurs between the two spheres. Before this breakdownoccurs, the stress on the specimen is essentially zero; however, theinstant the air gap breaks down, the applied voltage plus a radiofrequency signal developed by the arc is impressed across the specimenelectrodes and is maintained for 1 to 5 seconds, so that a tree will beinitiated in the composition being tested.

After the tree has been initiated, the SPINGS is held withoutapplication of voltage for about 6 days, and then 12,000 volts isapplied between the electrodes (an average applied voltage of 3000V/mm). Failure is indicated by dielectric breakdown. When failureoccurs, a sudden increase in current trips a relay that in turnterminates the test on that SPINGS and produces a signal on an eventrecorder. Individual SPINGS fail at different times. A failure time of agiven composition is expressed as (t₅₀) which is the time at which 50%of the SPINGS under test have failed. This characteristic property isalso called the electrical endurance of the composition.

This test has recently been standardized and is listed in the 1980Annual Book of ASTM Standards (American Society for Testing andMaterials, Philadelphia) as Standard D-3756-79. However, the timebetween tree initiation and actual test is not specified there.

This invention is now illustrated by the following examples of certainpreferred embodiments thereof, wherein all parts, proportions, andpercentages are by weight, unless otherwise indicated.

Unless otherwise specified, in all the tests in which polyethylene orpolypropylene was used polyethylene was a high pressure, low density(0.918 g/cm³) homopolymer, while polypropylene was an isotactichomopolymer having a density of about 0.902 g/cm³.

CONTROL 1

Ten SPINGS were molded at 180° C. from a blend of 97 percentpolyethylene and 3 percent polypropylene and tested at 12 KV asdescribed above, except that a 2 mm needle gap was used. The electricalendurance (t₅₀) was 16 minutes.

CONTROL 2

Ten SPINGS were formed as in Control 1, and tested according to theabove test method with a 4-mm needle gap. The electrical endurance (t₅₀)was 26.2 hours.

EXAMPLE 1

Tetraoctyl pyromellitate, 56 g, which had a melting point of 28° C., wasadded to 1760 g of pellets of a 97:3 polyethylene/polypropylene blend ina 3.79-L jar. The jar was sealed with a screw cap and rolled for 24hours to coat the pellets. The composition was extruded at 180° C. in a28 mm Werner & Pfleiderer twin-screw extruder to give pellets containingabout 3 percent of the ester in the polyethylene/polypropylene blend.The pellets were formed at 180° C. into SPINGS and tested as describedin Control 1. The test was discontinued after 1176 hr at 12 KV, duringwhich time no failure occurred.

EXAMPLE 2

Tetraoctyl pyromellitate, 42 g, was added to 1816 g of the samepolyethylene/polypropylene blend as that used in Example 1. The mixturewas rolled in a 3.79-L jar for 16 hrs, then was extruded at 180° C. inthe same 28 mm extruder; the extrudate was cut into pellets, which weredivided into two approximately equal portions. Each half was separatelyshaken in the original jar for one-half hour and re-extruded. Thiscomposition was formed at 180° C. into SPINGS, which were tested at 24°C. according to the method of Control 2. These SPINGS survived 600 hrwithout failure, at which point the test was terminated.

EXAMPLE 3

Trioctyl mellitate, 60 g, was added to 1760 g of a blend ofpolyethylene/polypropylene in a 3.79-L jar. This mixture was rolled fortwenty hours then extruded at 180° C. in the same extruder and theextrudate cut into pellets. This composition was formed into SPINGS at160° C. and tested according to the method of Control 1. These SPINGSsurvived 1056 hours without a failure, at which point the test wasterminated.

EXAMPLE 4

A mixed ester prepared by esterification of pyromellitic dianhydrideusing equimolar amounts of n-hexyl and n-octyl alcohols, was used inthis example. To 1816 g of a blend of polyethylene/polypropylene resinwas added 44 g of the mixed ester. This was rolled in a 3.79-L jar for20 hours. The mixture was then extruded at 180° C. and the extrudatepelletized. The pellitized material was added back into the jar andrerolled to pick up any residual ester. It was then reextruded andrepelletized. SPINGS were prepared from this composition at 180° C. andtested as in Control 2. These SPINGS survived 650 hours without afailure, at which point the test was terminated.

EXAMPLE 5

Polyethylene homopolymer, 2450 g, having a density of 0.920 g/cm³ and amelt index of 2.5 g/10 min, was blended in a Banbury mixer with 50 g ofthe same mixed C₆ and C₈ pyromellitate ester as used in Example 4. Whenthe temperature reached 149° C., the blend was shredded and cooled. Itthen was blended with 50 g of dicumyl peroxide at a temperature not over121° C. and again shredded and cooled. SPINGS were prepared by meltingthe material in a mold at 130° C., maintaining this temperature for 10minutes, applying a 13.8 MPa pressure, raising the temperature to 180°C., and maintaining it for 30 minutes to insure adequate crosslinking.The mold was cooled under pressure.

SPINGS prepared in this manner survived a 1200-hour test with a 4 mmelectrode gap without a failure.

WATER TREE TESTS

The effectiveness of the ester additives of the present invention ininhibiting water tree growth can be evaluated in a test similar to thatdescribed in U.S. Pat. No. 4,212,756 to Ashcraft et al. The polymericcomposition containing the inhibitor is shaped into a dish or "pieplate" having 24 conical depressions equally distributed in its bottom.The bottom of the dish is sprayed on the outside with metallic silver toform one electrode. After an electrolyte (e.g., 0.1% NaCl solution) ispoured into the dish, a wire connected to a high voltage source iscontacted with the solution. Tests are run at a voltage of 5 kV and afrequency of 1 kHz for a period of several days. The test plate is thencut into blocks containing one depression per block. The blocks are dyedwith methylene blue and cut into 0.4 mm slices parallel to the axis ofthe cone. The slices are examined under a microscope at a fixedmagnification and photographed.

The FIGURE shows the results of a test in which an uninhibitedpolyethylene was used as control. The inhibited sample contained 2% oftetrahexyl pyromellitate. It can be seen that water trees formed at theapex of the cone after 7,14, and 28 days are much smaller in the testspecimens of the inhibited composition. Tetrahexyl pyromellitate thus isan effective water tree inhibitor.

OVEN TESTS

Tree-growth inhibitors should not only survive the SPINGS test and showvery little tree growth in the water tree test but they should also havethe property of being retained in the insulating layer of the cable evenwhen the cable has been heated to excess. This is especially true forcrosslinkable insulating compositions since they are normally testedunder more severe heating conditions than the uncrosslinkedthermoplastics.

The test for inhibitor retention consists of pressing out small, 0.46 mmthick films for infrared scans. These films are mounted in standardstiff paper mounts and scanned in an infrared spectrophotometer over anappropriate frequency range which depends on the tree inhibitor beingtested. The mounted film is then hung in a circulating air oven at 75°C. The film is removed at intervals and scanned to determine the amountof inhibitor remaining. A good inhibitor will be substantially retainedeven after several hundred hours of tests. This test is much more severethan required to evaluate retention in normal cable service.

OVEN TEST EXAMPLES EXAMPLE 6 Tetraoctyl Pyromellitate in Polyethylene

Two films were formed from the same composition and mounted on infraredcard holders. The films were heated in a 75° C. high velocity air ovenand the infrared absorption peak at 1095 cm⁻¹ was used to determine theamount of tetraoctyl pyromellitate. The following tetraoctylpyromellitate concentrations were determined by this technique:

    ______________________________________                                        Oven                                                                          Time    Analysis #1   Analysis #2                                                                              Average                                      ______________________________________                                         0      2.91%         3.37%      3.14%                                         24 hr  2.93%         3.16%      3.05%                                         96 hr  2.99%         3.36%      3.17%                                        168 hr  3.08%         3.27%      3.17%                                        336 hr  2.96%         3.36%      3.16%                                        ______________________________________                                    

EXAMPLE 7 Trihexyl Trimellitate in Polyethylene

Two infrared films were prepared as in Example 6 and tested in the samemanner. In this example, the infrared peak at 1065 cm⁻¹ was used todetermine the amount of the ester present.

    ______________________________________                                        Oven                                                                          Time    Analysis #1   Analysis #2                                                                              Average                                      ______________________________________                                         0      3.04%         3.40%      3.22%                                         24 hr  3.24%         3.32%      3.28%                                         96 hr  3.13%         3.35%      3.24%                                        168 hr  3.22%         3.31%      3.27%                                        336 hr  3.03%         3.01%      3.02%                                        504 hr  3.11%         3.07%      3.09%                                        ______________________________________                                    

COMPARATIVE EXAMPLE 1 Dodecyl Alcohol in Polyethylene

Dodecyl alcohol, one of the alcohols listed in U.S. Pat. No. 4,206,260to E. J. McMahon, was found in this test to be readily lost from apolyethylene film under the conditions of Examples 6 and 7. The infraredpeak at 1060 cm⁻¹ was used in this study. The initial alcoholconcentration was 4.20-4.22%. After 4 hours at 75° C., thecharacteristic infrared peak disappeared, indicating complete loss ofdodecyl alcohol.

COMPARATIVE EXAMPLE 2 Acetophenone in Polyethylene

Acetophenone is recognized as an effective tree inhibitor. It is a majordecomposition product of dicumyl peroxide so that it usually is presentin dicumyl peroxide-crosslinked polyethylene compositions. The sametechnique was used for the film preparation as in the precedingexamples, but a temperature of 75° C. was maintained in the oven withoutair circulation. Infrared absorption at 955 cm⁻¹ was determined atintervals, giving the following acetophenone concentrations:

    ______________________________________                                        Oven        Acetophenone                                                      Time        Concentration                                                     ______________________________________                                         0          1.63%                                                             15 min.     0.49%                                                             30 min.     0.18%                                                             60 min.     0                                                                 ______________________________________                                    

It can be seen that acetophenone is lost very quickly under very mildconditions.

We claim:
 1. A tree-resistant composition for use in .[.electricalequipment.]. .Iadd.a power transmission cable.Iaddend., said compositionconsisting essentially of an ethylene polymer selected from the groupconsisting of ethylene homopolymers and ethylene copolymers with atleast one other ethylenically unsaturated monomer, ethylene beingpresent in such copolymers in an amount of at least 85 weight percent,and, as a tree-growth inhibitor, an effective amount of at least oneorganic carboxylic ester having at least one .Iadd.but no more than two.Iaddend.aromatic ring.Iadd.s .Iaddend.and at least three .Iadd.but nomore than four .Iaddend.carboxylic ester groups, .Iadd.at least onecarboxylic ester group being attached to one aromatic ring and theremaining carboxylic ester groups being attached to the other aromaticring, if present, which is fused to the first aromatic ring or joined toit by a single bond, an alkylene group, a carbonyl group, or a heteroatom, .Iaddend.said inhibitor melting below about 50° C. and beingliquid at the power transmission cable's intended operating temperature.2. A composition of claim 1 wherein the amount of the tree-growthinhibitor is about 0.25-5% based on the weight of the ethylene polymer.3. A composition of claim 2 wherein the amount of the tree-growthinhibitor is 1-3% based on the weight of the ethylene polymer.
 4. Acomposition of claim 1 wherein the ethylene polymer has a density ofabout 0.92 g/cm³ or less.
 5. A composition of claim 4 wherein theethylene polymer is a homopolymer.
 6. A composition of claim 1 whereinthere is present, in addition to the ethylene polymer, also apolypropylene or ethylene/propylene copolymer, the amount of suchadditional polymer or copolymer being up to about 10 weight percent ofthe ethylene polymer.
 7. A composition of claim 1 wherein thetree-growth inhibitor is selected from the group consisting of esters ofmellitic acid, trimesic acid, hemimellitic acid, trimellitic acid, andpyromellitic acid with a C₄ -C₁₂ aliphatic or cycloaliphatic alcohol. 8.A composition of claim 7 wherein the ester is tetraoctyl pyromellitate.9. A composition of claim 1 which also contains an organic peroxide. 10.A composition of claim 1 wherein the inhibitor is a mixed ester of apolycarboxylic acid with two or more alcohols.
 11. A composition ofclaim 1 which also contains a form of elemental carbon as a filler.